In a first kind of electrographic printing, particularly in the process of electrophotography, a light image of an original document to be copied or printed is recorded in the form of a latent electrostatic image on a photosensitive member. The generated electrostatic latent image is subsequently rendered visible by application of electroscopic particles, commonly called toner. The toner particles preferably have a definite electric charge sign and as such are attracted by the electrostatic charge pattern of opposite charge sign in proportion to the field strength of the respective areas defining the pattern.
The toner particles forming the visual image are then transferred from the photosensitive member to a support member or receptor support, such as a sheet of plain paper or a plastic film, further shortly indicated as "sheet". Since the toner image is then in a loose powdered form which may be easily disturbed or destroyed, it has to be permanently fixed or fused on said sheet in a fusing or fixing device.
In a second kind of electrographic printing, particularly in Direct Electrostatic Printing (DEP), electrostatic printing is performed directly from a toner delivery means, e.g. a magnetic brush assembly, on a receiving member substrate, called "sheet", by means of an electronically addressable printhead structure. Herein, the toner is deposited directly in an imagewise way on said sheet without occurrence of any latent electrostatic image. An overall applied propulsion field between the toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the toner-stream is modulated according to the pattern of potentials applied to the control electrodes. The deposition step is followed by a fusing step.
As a DEP device has already been described, e.g. in U.S. Pat. No. 3,689,935 (Pressman) and in EP-A-0 710 898 (Agfa-Gevaert N.V.), no further description is necessary in the present application.
In order to permanently fix a toner image to a sheet, it is well known in the art to apply thermal energy. By elevating the temperature of the toner material to a point at which the constituents of the toner coalesce and become tacky or melt, the toner is absorbed into the fibres of the sheet or fixed to the substrate. As thereafter the toner cools, solidification causes it to be firmly bonded to the sheet.
Several approaches to thermal fusing of electroscopic toner images are known from the prior art. Special attention has to be focused on the production of duplex or recto/verso copies or prints, i.e. copies where images are formed on both sides of the sheet.
The production of duplex or recto/verso copies poses problems due to a severely occurring offset problem, which will be discussed in great detail on the next pages.
Duplex printing in electrographic systems, e.g. in electrophotographic copiers, working according to the two pass method may be carried out in one of the following ways.
(i) A so-called "manual two pass method" that requires manual re-feeding of multi-layer imaged simplex sheets, e.g. colour imaged simplex sheets. That is, after the first side of a sheet is imaged and fused, the sheet is transported to an output tray. Then, the operator places this sheet back in one of the input trays, upon which the sheet is again passed through the engine. This time an image is transferred and fused onto the opposite side of each sheet having an image on a first side.
(ii) A so-called "automated postponed two pass method", that requires the collection of simplex sheets in a duplex tray. That is, after the first side of a sheet is imaged and fused, the sheet is transported to a duplex tray inside the engine. After the last sheet in a set has been received in this duplex tray, all sheets are again passed automatically through the imaging device. This time an image is transferred and fused onto the opposite side of each sheet having an image on a first side.
(iii) A so-called "automated immediate two pass method" that requires reversing the simplex sheets immediately after fusing and interleaving them with sheets receiving the first image on the first side in order to receive an image on the opposite side.
These two-pass duplex methods have some very important drawbacks, usually related to the twofold passing through the fuser.
(i) Two passes through the fuser require more energy than one pass. This is especially important for the case of multi-layer imaging, e.g. colour imaging, with its high energy requirement for thorough fusing and mixing of the respective layers or colours.
(ii) At the same time the fuser needs to operate at twice the speed of the duplex throughput, which again in the case of multi-layer or colour fusing is not at all straightforward.
(iii) The change in moisture content (say about 30%) between the first and the second imaging pass results in an image quality that is not equal between the first side imaging and the duplex side imaging.
(iv) In addition, this change in moisture content also alters the mechanical properties of the paper, which--combined with the additional complexity of a duplex paper path--results in a highly increased risk for jams in duplex printing.
(v) Because of the need for a release agent (e.g. silicon oil) in hot roller fusing, silicon oil remaining from the first pass colour imaging may contaminate the image forming elements, resulting again in non constant image quality over time, with possible effects such as image smearing etc.
(vi) Excessive paper curl is not only troublesome in the processor but also extremely difficult to handle in output stackers and finishing devices.
In other prior art systems, also single pass duplex copying has been disclosed. Three methods are known in the art.
(i) According to a first method, first and second images are formed sequentially on a photoreceptor. The first image is transferred from the photoreceptor to the first side of a receptor sheet. Then the sheet is stripped off the photoreceptor, inverted while the first image remains unfixed, and then the second image is transferred to the second side of the receptor sheet. Both images are then fixed onto the receptor sheet in a suitable fuser.
(ii) Other single pass duplex printing methods use intermediate image carriers, e.g. a belt or a drum. The first and second images are sequentially formed on a photoreceptor. The first image is transferred to an intermediate image carrier. The receptor sheet is then passed between the photoreceptor and the intermediate image carrier. The receptor sheet is then simultaneously receiving first and second images.
(iii) Other systems deal with "single pass duplex" methods employing two photoreceptors and two exposure systems. A first image is deposited on one photoreceptor and a second image is deposited on the other photoreceptor. These systems are considered the ultimate duplex throughput systems since they produce twice the number of images of "two pass duplex" systems at equal process speed.
Many problems exist with the traditional single pass duplex systems.
(i) One problem is in conveying the duplex receptor sheet to the fuser. In particular, the receptor sheet with the two unfused images on opposite sides, must be transported from the toner transfer station to the fuser. Preferably this is not done with a conventional transport since the transport would make contact with one of the sides of the receptor sheet and smear the unfused toner image. Also, to avoid the leading edge of the sheet from downwards deviating from the path between transfer station and fuser station, it is preferred that this path is very short. Thereto, the fuser must be very close to the photoreceptor. This creates problems in mechanical mounting, problems due to unwanted heating the photoreceptor and problems of contaminating the photoreceptor with fuser release materials, e.g. silicone oil vapour.
(ii) In addition there is the problem of the rather uncontrollable velocities of sheets passing through roller fusers. There seems to be an obvious need to accurately match the velocity of the receptor sheet transport with the velocity of the photoreceptor to prevent "skips" and "smears" during transfer. Furthermore, for high resolution digital printing, excessive instantaneous photoreceptor velocity variations (cfr. "jitter") cannot be tolerated. Even in conventional copiers it is preferable to keep the fuser rollers one sheet length away from the transfer zone. For these reasons it is desirable to thermally insulate and mechanically isolate the photoreceptor transfer zones from the hot fuser rollers.
(iii) Single pass duplex systems using more than one photoreceptor and more than one exposure system, generally require web paper feed in which the copy is wound up on a roller or cut into individual sheets after fusing. This, unfortunately, introduces additional components and complexity into the system. It is, therefore, also desirable to provide a single pass duplex system having a discrete receptor sheet feed system rather than a web paper feed system.
(iv) Moreover, in high quality copying and printing it has to be made sure that both sides of the duplex imaged sheets experience substantially the same "fusing history", referring namely to the temperature and pressure trajectory.
Multi-layer electrographic printing, e.g. multi-colour electrophotographic printing, may seem equivalent to multiple monochrome (commonly black and white) printing of various toner layers. Yet, successive part images have to be recorded in superposition. These successive part images may comprise a superposition of different toner separation images. In one embodiment, the traditional colour components cyan C, magenta M and yellow Y, are augmented with at least one extra colour component according to one toner type. This extra colour component may have another density or colouring power (obtained by a different degree of pigmentation) of either cyan, magenta or yellow. In another embodiment, a traditional black component K is added to the three usual colour components. In another embodiment, for each traditional colour component, CMY or CMYK, at least a second colour component, having a lower pigmentation level, C'M'Y'(K') is added. According to another embodiment, some tone levels of the original image are reproduced by applying two different toners, having substantially the same chromaticity, or more specifically by applying two achromatic toners, i.e. greyish or black toners of which the chromaticity is substantially zero.
In one embodiment each single toner image is transferred to the receptor sheet in superimposed registration, thereby creating a multi-layered toner image on the receptor sheet. Thereafter, the multi-layered toner image is permanently fixed to the receptor sheet creating a multi-layer or colour copy or print. Whereas the fixing of monochrome toner images does not raise major problems in practice, the fixing of multi-layer or colour images is much more difficult. We will base the discussion on colour images, which are a specific case of multi-layer images.
(i) As a colour toner image intrinsically is thicker than a monochrome toner image, for a same print-quality and a same print-throughput, the supply of fusing heat has to be increased and even controlled more stringently.
(ii) The increased amount of toner requires a longer fusing time demanding a nip with a larger length or a slower rotation of the fuser rollers. It may be remarked that the nip between both rollers, more exactly between the resilient coverings of these rollers, is in fact the area where heat and pressure initiate the fusing and thus the fixing of the toner image on a sheet conveyed between the rollers.
(iii) The fixing of multi-layer images is also difficult as compared to the prior art of fixing single layer images, in that it needs a strongly different geometry of the fixing rollers, calling for a dedicated design of the kind and the geometry, e.g. thickness of the resilient layer on each roller, the diameter of the rollers, the pressure applied to the rollers, etc.
In view of the many problems described, a very interesting application comprises U.S. Pat. No. 4,427,285. However, some drawbacks still pose severe restrictions to the effective use of said patent.
A first restriction of the solution disclosed in U.S. Pat. No. 4,427,285 is that it is not intended for and hardly can be applied for fusing multi-layer toner images.
A second and important restriction of U.S. Pat. No. 4,427,285 is that its solution needs heat isolation means between the fusing station and the photoreceptor.
Hereto, it discloses e.g. a transport mechanism for conveying a receptor sheet having toner images on both sides, towards the heat source for fusing, thereby thermally isolating the photoreceptor from the heat source.
U.S. Pat. No. 4,427,285 also discloses a heat shield disposed between a transfer station and a heat applying device, thereby carrying out two distinct functions, namely
(i) isolating the heat, and
(ii) tacking the unfused images onto the receptor sheets.
More particularly, it discloses the use of compacting rollers, which have to fulfil both said functions of thermal isolating and tacking.
As will be clear from the detailed description, it is a remarkable advantage of the present invention that no initial tacking down is necessary and that no compacting rollers are necessary.
It will also become clear from the detailed description, that no intermediate fusing is necessary. Such an intermediate fusing inevitably would increase the construction-cost of the apparatus, and could reduce the reliability of the system, as the dimensional stability of the sheets would diminish because of changing moisture content.
In view of the above, fusing stations of the type described above are unsuitable for being installed in electrographic apparatus designed for single-pass fusing of sheet-fed multi-layer or colour duplex copies.